Optical imaging can provide valuable information about turbid media such biological tissue. Recent developments in both hardware and software enable rapid acquisition and processing of optical data to generate optical images of tissues. The use of optical imaging of living tissue, such as breast, brain or whole body of small animals, is growing within the medical and pharmaceutical research communities. Its advantages over other imaging modalities, such as X-ray, ultrasound, PET or SPECT and MRI, is that it can provide rich optical spectrum analytical information about tissue composition and that the imaging is done using non-ionizing radiation (i.e. light) without any adverse effect on tissue. For example, chromophore information can help discern between oxygenated and deoxygenated blood that is quite useful to understand the function within the tissue. In some cases, an exogenous marker, whether fluorescent or a chromophore, may be injected into the tissue to aid in localizing or visualizing objects of interest. Markers can selectively attach to certain molecules within tissue and the concentration of a marker within tissue can reveal important information about the state of the tissue.
Because tissue is a turbid medium, namely it scatters light heavily, optical imaging is a challenge. Optical scatter in tissue largely results from changes in the index of refraction caused by cellular and intracellular boundaries. Injected light thus becomes a diffuse glow when detected either at the other side of the tissue in transmission mode or at the same side of the tissue in reflection mode. In the imaging process, scattering of light within the tissue must be accounted for correctly if imaging with good spatial resolution is to be achieved. When light is injected into tissue, it is scattered and absorbed. The combination of the scattering and absorption of the light provides the overall attenuation of light between source and detector. In the case of a fluorophore, the absorbed light may be reemitted at a wavelength and time that varies as a function of the fluorophore properties.
Optical scatter, namely the density and level of contrast of index of refraction boundaries within tissue, is generally a source of structural information. However, since the absorption and/or the fluorescent reemission is a source of biological information of interest that is not obtainable with X-ray imaging, and since the location within the tissue of this biological information is to be identified, optical scatter is determined within the imaging process to allow for proper spatial identification of concentration of fluorophore and/or chromophore concentrations. Generally, scatter information is obtained by acquiring time dependent optical information, namely through time domain or frequency domain optical data acquisition.